SELECT

select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO ? synchronous I/O multiplexing

#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);

FD_CLR(int fd, fd_set *set);
FD_ISSET(int fd, fd_set *set);
FD_SET(int fd, fd_set *set);
FD_ZERO(fd_set *set);

select waits for a number of file descriptors to change status.

Three independent sets of descriptors are watched. Those listed in readfds will be watched to see if characters become available for reading, those in writefds will be watched to see if it is ok to immediately write on them, and those in exceptfds will be watched for exceptions. On exit, the sets are modified in place to indicate which descriptors actually changed status.

Four macros are provided to manipulate the sets. FD_ZERO will clear a set. FD_SET and FD_CLR add or remove a given descriptor from a set. FD_ISSET tests to see if a descriptor is part of the set; this is useful after select returns.

n is the highest-numbered descriptor in any of the three sets, plus 1.

timeout is an upper bound on the amount of time elapsed before select returns. It may be zero, causing select to return immediately. If timeout is NULL (no timeout), select can block indefinitely.

On success, select returns the number of descriptors contained in the descriptor sets, which may be zero if the timeout expires before anything interesting happens. On error, ?1 is returned, and errno is set appropriately; the sets and timeout become undefined, so do not rely on their contents after an error.

EBADF

An invalid file descriptor was given in one of the sets.

EINTR

A non blocked signal was caught.

EINVAL

n is negative.

ENOMEM

select was unable to allocate memory for internal tables.

Some code calls select with all three sets empty, n zero, and a non-null timeout as a fairly portable way to sleep with subsecond precision.

On Linux, timeout is modified to reflect the amount of time not slept; most other implementations do not do this. This causes problems both when Linux code which reads timeout is ported to other operating systems, and when code is ported to Linux that reuses a struct timeval for multiple selects in a loop without reinitializing it. Consider timeout to be undefined after select returns.

#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int
main(void)
{
    fd_set rfds;
    struct timeval tv;
    int retval;

   /* Watch stdin (fd 0) to see when it has input. */
    FD_ZERO(&rfds);
    FD_SET(0, &rfds);
    /* Wait up to five seconds. */
    tv.tv_sec = 5;
    tv.tv_usec = 0;

   retval = select(1, &rfds, NULL, NULL, &tv);
    /* Don’t rely on the value of tv now! */

   if (retval)
        printf("Data is available now.\n");
        /* FD_ISSET(0, &rfds) will be true. */
    else
        printf("No data within five seconds.\n");

   exit(0);
}

4.4BSD (the select function first appeared in 4.2BSD). Generally portable to/from non-BSD systems supporting clones of the BSD socket layer (including System V variants). However, note that the System V variant typically sets the timeout variable before exit, but the BSD variant does not.

accept(2), connect(2), poll(2), read(2), recv(2), send(2), write(2)